GLUT supports simple cascading pop-up menus. They are designed to let a user
select various modes within a program. The functionality is simple and
minimalistic and is meant to be that way. Do not mistake GLUT's pop-up menu
facility with an attempt to create a full-featured user interface.

Create a new pop-up menu for the current window, attaching it to the
given mouse button. A previously attached menu (if any), is detached before
and won't receive callbacks anymore.

It is illegal to call attachMenu while any (sub-)menu is in use, i.e.
popped up.

X Implementation Notes: If available, GLUT for X will take advantage of
overlay planes for implementing pop-up menus. The use of overlay planes can
eliminate display callbacks when pop-up menus are deactivated. The
SERVER_OVERLAY_VISUALS convention is used to determine if overlay visuals
are available.